I’ve been recently with my DD. I was nervous as first trip without DH or friends in years. It was so easy. We arrived at 8pm on Eurostar and found the metro easily, it felt very safe. DD navigated round with a Dorling Kindersley Top 10 Paris book and pull out map. Everything felt walkable but we did get the metro a couple of times and it was fine.
I think a really key thing to feel safe as a solo traveller is booking a hotel in a safe neighbourhood that gets good reviews. Once you are there, the hotel staff can help with recommendations for transport, eating out etc. I chose a hotel over air bnb for that reason, also because I’ve had some iffy air bnb check ins from time to time and I didn’t want to risk arriving and not being able to get in.
I did split cards up and keep passports in hotel safe and had travel passes saved to both of our phones just in case one of us lost a phone/bag or had one stolen but it was fine. We weren’t hassled anywhere.
The little bistros are great for dinner, we enjoyed eating out in the ones in the quieter neighbourhood we were staying in. The one day we ate out by the Seine we had rude service from waiter and not great pizza but what do you expect from a tourist hotspot.
I researched places to eat in advance for evenings on Google maps to check for good reviews and saved them, meant I could do that at my leisure and have a few options up my sleeve, that helped me feel organised and calm.
Sometimes if I am anxious it helps me to write in my notes app on my phone an outline plan for the day with lunch / cafe / dinner options and transport details. Not very spontaneous but it keeps me calm and I quite enjoy planning trips anyway! Maybe it would help you?
Enjoy, it’s such a beautiful city!